Glycogen is a homopolysaccharide consisting of glucoses like starch, which is a storage polysaccharide in plants. Glycogen has a polymer structure in which D-glucoses are linked together with α1-4 glycoside linkages and a highly branched structure which has a branch per 8 to 10 glucose residues due to α1-6 glycoside linkage.
Glycogen is known as an animal storage polysaccharide. In animals, glycogen is contained in a granule form (glycogen granule) in almost all cells, particularly in liver and muscle in a large amount. Muscle glycogen is an energy source for muscle contraction. Liver glycogen is used to maintain blood sugar during fasting. The difference in the characters of glycogens corresponds to the difference in the functions thereof. The muscle glycogen has a molecular weight of 1-2 million. The liver glycogen has a molecular weight of 5-6 million and sometimes as much as 20 million (Iwanami Seibutsugaku Jiten [Dictionary of Biology], 4th Ed., p. 354, Iwanami-shoten (Tokyo) published on Mar. 21, 1996).
While glycogen is an animal storage polysaccharide, the glycogen is known to have an action of enhancing liver function. It was reported that a glycogen extracted from cuttlefish and scallops has a potent anti-tumor activity (Yosiaki Takata, et al., J. Mar. Biotech., 6. pp. 208-213 (1998)). Such a glycogen is useful as a novel material for functional foods and its applications have been developed.
Glycogen is biosynthesized from monosaccharides, such as glucose or the like, in the animal body.
In general, a polysaccharide sugar chain is synthesized by a chemical method or an enzymatic method. Both the methods are based on the principle that the OH group at an anomer position, which is to form a sugar hemiacetal ring, is previously activated as a leaving group and is subsequently replaced with another sugar or a biological component. To date, sialyl LeX ganglioside (contributing to the epoch-making result of researches on cell adhesion molecules or cancer-related antigens), calicheamicins (having a carcinostatic action), and the like have been developed using the chemical method, and cyclodextrins (having an inclusion action), coupling sugars (sweeteners substituting for sucrose, which are less responsible for tooth decay), and the like have been developed using the enzymatic method.
In the chemical method for synthesis of polysaccharide sugar chains, for example, it is known that a long chain oligosaccharide is degraded with acids; and various resultant monosaccharides are treated with a dilute acid, resulting in the reverse reaction in which a mixture of oligosaccharides are produced. In the enzymatic method, it is known that when sucrose is treated with invertase, which is a sugar hydrolytic enzyme, at a high concentration and high temperature, fructose is transferred to glucose, with 1 to 3 fructose molecules per one sucrose molecule to produce a fructooligosaccharide.
In general, chemical synthesis of polysaccharide sugar chains requires sugar donors, sugar acceptors, and promoters. Further, the sugar donors and the sugar acceptor or optionally their derivatives have to be prepared; and factors, such as solvents, dehydrators, temperatures, and the like have to be determined stringently. Furthermore, complicated steps, such as conversion or elimination of protecting groups, liberation of a specific hydroxyl group, or the like, are required. Therefore, it is not easy to chemically synthesize polysaccharide sugar chains. The sugar donors and the sugar acceptors as well as the factors have to be determined separately for each different substrate. There was no known technique for synthesis of sugar chains which can be generally applied to various substrate materials irrespective of their types.